1. Field of the Invention
This invention relates to a semiconductor storage apparatus and more particularly to a storage apparatus which comprises a volatile semiconductor memory and a non-volatile semiconductor memory as a backup device for saving data.
2. Explanation of the Prior Art
Conventionally, storage apparatus used for information processing systems such as computer systems or electronic switching systems are divided into two types: primary storage apparatus constituted from a random access memory (RAM) connected to a central processing unit by way of a bus, and secondary storage apparatus such as magnetic disk apparatus, magnetic tape apparatus or optical disk apparatus for a backup purpose. Generally, a primary storage apparatus is constituted from a volatile random access memory which can be accessed directly at a high rate of speed from a central processing unit as opposed to a secondary storage apparatus having a large capacity and non-volatility and can be characterized by magnetic disk apparatus, magnetic tape apparatus, optical disk apparatus or electronic disk apparatus. The primary storage apparatus and the secondary storage apparatus have a mutually complementary relationship, and it is the current general trend to configure systems using both types of storage apparatus.
Of the conventional storage apparatus described above, the primary storage apparatus has a problem in that it requires a countermeasure to compensate for the volatility of the stored information. Of the secondary storage apparatus, those storage apparatus represented by a magnetic disk apparatus, a magnetic tape apparatus or an optical disk apparatus that require mechanical operation are disadvantageous in that a delay time caused by a mechanical operation, a seek time and a search time are involved, resulting in a low access speed. Although an electronic disk apparatus employing a semiconductor memory has a high access speed, it is disadvantageous in that a battery of large capacity is required as a countermeasure for the volatility of the stored information, resulting in an increase in weight and size and is further disadvantageous in that it requires a tertiary storage apparatus for holding the stored contents for a long period of time.